1. Field of the Invention
The invention relates generally to the field of devices for measuring the osmotic pressure of human body fluids, and in particular to apparatus for measuring, in vivo, the osmolarity of tears.
2. Description of the Related Art
An osmometer is a device for measuring osmotic pressure. The osmotic pressure of a solution is defined as the excess pressure that must be applied to the solution to prevent the passage into it of solvent that is separated from the solution by a semipermeable membrane which allows only solvent molecules to pass through. The osmolarity of a solution is a characteristic determined by the ionic concentration of a dissolved substance per unit quantity of solvent.
The importance of the osmotic pressure of various body fluids relates to correlations that have been sometimes noted between the osmolarity of a particular fluid and some pathological condition of the body. For example, the relationship of the osmotic pressure of tears to various pathologic conditions is discussed in the article "The Total Osmotic Pressure of Tears in Normal and Various Pathologic Conditions," by Gary J. Mastman et al., in ARCHIVES OF OPHTHALMOLOGY, Vol. 65, April, 1961, pages 71/509-75/513.
The particular pathologic condition designated "dry eye" and its connection to tear film osmolarity is described in the article "Osmolarity of Tear Microvolumes in Keratoconjunctivitis Sicca," by Jeffrey P. Gilbard et al., in Arch. Ophthalmol., Vol. 96, April, 1978, pages 677-681. When the surface of the eye starts to dry out the tear film becomes hypertonic (elevated osmolarity), causing discomfort and epithelial damage.
Presently measurements are conventionally made in vitro by removing a tear sample from the eye using a micropipette and measuring the depression of the freezing point that results from heightened osmolarity. This technique is difficult to carry out and the results are plagued by many sources of error. It would be a great advance in the art of ophthalmological diagnostic testing if it were possible to measure, in vivo, the osmolarity of tears easily and accurately.
The osmolarity of sweat has been linked to cystic fibrosis. Measurements of the electrical conductivity of sweat in vitro have been used in the diagnosis of this condition in children. Information about this topic can be found in the article "Electrical Conductivity of Sweat--A Simple Diagnostic Test in Children," by H. Shwachman et al., in the journal PEDIATRICS, July 1963, pages 85-88. A sweat conductivity analyzer for in vitro measurements is produced commercially under the trademark Sweat.Chek by Wescor, Inc., 459 South Main Street, Logan, Utah 84231. It would be an important development in the art of medical diagnostic testing if it were possible to measure, in vivo, the osmolarity of sweat easily and accurately.